This invention relates to a control system for a pyrolysis device of the type used for volatilizing and burning organic material from a metal part to which the organic material is bonded. Incineration occurs in a zone adjacent to the device's main chamber in which the material is volatilized, resulting in innocuous products of combustion which may be discharged into the atmosphere. Such an incineration zone is typically provided by an afterburner chamber in which an afterburner, positioned downstream of the device's main chamber in which pyrolysis occurs, burns the volatilized organic material (referred to herein as "vapor"). The remaining metal part is reclaimed for reuse because the cost of reclamation is less than that of making the metal part anew. Such reclamation by pyrolysis has evolved into a subindustry of considerable economic significance not only because pyrolysis is cost-effective, but because incineration of the vapor of polymeric materials which are not economically recyclable, conveniently and beneficially disposes of them.
The vapor to be incinerated is generated when mounting means for engines and electric motors (collectively referred to as "motor mounts"), and similar steel parts bonded to rubber; or, copper-containing electrical parts such as armatures, stators, transformers and the like; or, painted ferrous or non-ferrous steel parts; or, metallic bodies of arbitrary shape which are coated with, or bonded to polymeric materials (referred to herein as "polymer-bonded metal parts"), are to be pyrolized in a pyrolysis furnace.
Polymeric materials to be disassociated from metal parts are such materials as are commonly bonded to a metal substrate or matrix and include natural and synthetic elastomers; for example, natural rubber and synthetic rubber which are polymers of dienes; silicones which are polymers of siloxanes and the like; and, natural and synthetic resinous materials including natural shellac and synthetic plastics such as phenolics and acrylics, particularly paints.
The foregoing polymeric materials are to be separated from the metal matrix to which they are bonded without melting the metal, and preferably, in most instances, without causing warpage or other undesirable deformation of residual metal matrix. It is self-evident that such separation may be effected by directly incinerating the polymeric materials, as is typically done in an incinerator for waste, but it is equally self-evident that the requirement of incineration without damaging the metal parts will not be met. Of course, damage to the parts can be minimized if only a few parts are incinerated together, but this method is undesirable because it does not lend itself to reclaiming a large enough mass of parts to be economical.
The term "pyrolysis oven" has been used in the art to indicate that there is no incineration of organic material on the metal parts within the oven's main chamber. The material is simply volatilized (or vaporized) without being burned in the oven's main chamber. The vapors are then burned in the afterburner chamber. Such operation of a "pyrolysis oven", where there is no fire in the main chamber, is supposed to clearly distinguish its function, from that of a "pyrolysis furnace" in which there is. Nevertheless, the terms are often misused or interchanged, particularly in relation to devices using an afterburner in an afterburner chamber of the furnace, with no thought given as to the significance of where the fire is maintained. This invention is specifically directed to a pyrolysis furnace with an afterburner chamber in which furnace a fire is sustained in the furnace's main chamber, under controlled temperature conditions, such that an explosion does not occur. A charge of metal parts on a cart is charged to the main chamber, the charge is brought up to ignition temperature, ignited, and the fire sustained under controlled conditions until the charge is burned out.
It is known that heating of the metal parts to 700.degree.-800.degree. F. in an enclosure with limited air intake will char or degrade all known combustible contaminants without ignition if the percentage of contaminants is less than about 2% by weight ("wt") of the parts, but we are concerned with higher amounts of combustibles in the range from 2% to about 40% by wt. To the extent that our pyrolysis furnace is used to reclaim metal parts associated with a minor proportion by wt of polymers, it is unlike a typical incinerator in which a major proportion by wt of the load is consumed by incineration.
In pyrolysis ovens belonging to the class including a main burner and an afterburner, it is desirable to process a large mass of metal parts with the concomitant generation of a large volume of combustible vapor. This requires that one avoid a fire in the main chamber, and that the amount of vapor be generated at a controlled rate so that it can be incinerated in the afterburner chamber without triggering an explosion.
Since in either an oven or a furnace, the hot air to vaporize the organic material is generated by a natural gas, LP gas or oil-fed burner (referred to as the "main burner") with a shielded flame, there is always a high probability that if the material is vaporized too fast to be incinerated in the afterburner, the vapors will ignite and cause an explosion in the main chamber. It is such an explosion which must be avoided. To do so, that is avoid the explosion, the obvious thing to do is to avoid an igniting spark or fire in the main chamber. We do not do so. By sustaining a fire in the main chamber, it was hoped we would only have to contend with excessive temperature, due to the burning of too much vapor. Since ignition had already occurred, an explosion seemed most unlikely. But the explosions occurred. We were unable to prevent them. The precise mechanism of the explosion and the criticality of vapor build-up is not understood, but we have found that, even while deliberately sustaining a fire in the main chamber, we can still set off an explosion if a critical build-up of vapor is exceeded.
It is unnecessary to point out that, when operating under near-explosive conditions, a very small misstep can set off the explosion, and any control system which prevents such an explosion from being set off acquires great merit.
Numerous control systems have been used in the past to attempt to ensure the safe operation of furnaces and dryers in which flammable volatiles are generated. One of the methods of preventing a fire or explosion within the main chamber is to equip it with water nozzles which are actuated by a temperature sensing means situated so as to sense the temperature within the main chamber, and when a preset temperature is exceeded, a signal is generated which results in spraying water onto the mass being burned, to help put out the fire and cool the mass.
For example, U.S. Pat. Nos. 3,767,179 and 3,839,086 to Larson teach a control system for a rotating drum dryer for metal scrap contaminated with flammable oil. Sufficient oil is evaporated to bring about at least partial combustion of the oil and maintain a self-supporting flame. The dryer is provided with a bank of water-injection nozzles within the dryer which are individually controlled by cam-actuated metering valves, to inject varying amounts of water into the dryer depending upon the temperature within it. A signal generated by a thermocouple situated within an exhaust gas stream from the dryer, determines the number of nozzles which are simultaneously activated in order to avoid oxidizing, fusing or even melting of the scrap. Since partial combustion is maintained, problems of varying severity within the dryer are quenched proportionately, at the same time avoiding an explosion.
A reclamation oven with a control system for preventing fires and explosions and thus controlling excess temperature within it, is disclosed in U.S. Pat. No. 4,270,898. The fire and explosion control method senses a fire situation before it occurs, and keeps the fire from happening by instituting a timely extinguishing system. A thermocouple is installed in the exhaust, downstream from the afterburner, and when the temperature exceeds a preset temperature, a signal from the thermocouple actuates an automatic valve assembly to open it and spray water onto the too-hot parts in the main chamber. When the parts cool sufficiently, the valve assembly closes. The system prevents fires and explosions and thus controls excess temperatures. The main burner is not shut off when the water spray comes on, though the main burner goes off when the oven reaches the set-point temperature, nor is the average temperature above the metal parts in the oven's main chamber (referred to as the "ambient temperature" in the main chamber) monitored. The prior art system in which a fire in the main chamber is prevented, is quite unlike our system in which a fire is maintained under conditions imposed by controlling temperature, so that an explosion is obviated.
Another system relating to incineration of unwanted organic material such as oil associated with metal parts, particularly scrap or swarf, is disclosed in U.S. Pat. No. 3,705,711 to Seelandt et al. Only as much air and fuel as is required to fuel the main burner, is burned to minimize oxidation of the metal parts and to minimize the risk of explosion. Control is provided by limiting the amount of combustion air to the main chamber when a preset pressure is exceeded. It is suggested that the temperature within the drum may first be lowered by throttling back the main oil burner or by stopping the feeding of metal scrap into the dryer drum. When the main burner output is reduced to its lower limit and the temperature within the drum is still too high, a water spray may be actuated. Should the spray be insufficient to lower the temperature, the feeding of the scrap into the drum is reduced or stopped. The problem is that the time period required for these operations is much longer than that permitted by conditions under which an explosion occurs because of ignition of the built-up vapor. As a result, such a system is wholly unsatisfactory under the conditions of operation of a pyrolysis furnace.
The control system of our invention is a two-stage system which, in the first step, shuts off the main burner with a signal from a first thermocouple in the exhaust gas stream downstream of the afterburner; if necessary, in the second step, either one of a second or a third thermocouple located upstream of the afterburner, determines whether the water spray is to be actuated. If sufficient cooling of the gases in the main chamber occurs without actuating the water spray, there is a substantial savings in fuel costs. Shutting off the main burner avoids the use of water to cool the parts which are at the same time being heated by a burner which is not shut off. The efficacy of our system is predicated on the discovery that there is a significant difference in temperature between the ambient temperature in the main chamber and the temperature of the gas stream in the throat of the main chamber just prior to entering the afterburner. It was always known that there is a large difference in temperature between those in the main chamber and the exhaust stack downstream of the afterburner.